Of the most widely used industrial processes for producing silicon-containing ferro alloys there can be named three:
1. A metallothermic process wherein an alloying element is reduced by aluminium and/or by ferro silicon made by electrolysis or electric smelting respectively in individual furnaces.
A disadvantage of this process resides in the fact that smelting is carried out periodically in two or three stages with the use of high cost reducing agents. Moreover, the application of this process involves considerable losses (up to 50%) of the alloying elements with slag.
Reprocessing of the slag is a complicated scientific and technological problem.
In addition, the slags formed in the process of casting, for instance alloys with barium, contain toxic compounds, and they have to be kept in special containers.
2. A carbothermic two-stage process, wherein silicon and the alloying elements are reduced by a carbonaceous reducing agent such as coke or coal, in the presence of a semi-product obtained in another smelting furnace and containing alloying element carrying carbides.
A disadvantage of this process resides in the fact that the reduction is carried out in two stages. Preserving and reprocessing of carbides contained in the semi-product, such as carbides of rare earth and alkaline earth metals, involve considerable difficulties, unsatisfactory production conditions (fire safety and explosion proofness), and a rather high consumption of energy and materials.
3. A carbothermic one-stage process, wherein the reduction of silicon and alloying element is carried out in one smelting furnace. Theoretically, this process is the most feasible of all the three. However, numerous attempts of the application thereof for smelting certain alloys containing rare earth, alkaline and other metals difficult to reduce, met considerable difficulties in connection with slagging or formation of deposits in a furnace smelting chamber. This resulted in a sharp deterioration of the alloy quality as well as of all production and economic figures.
To hinder the reactions causing an accumulation of carbides in the furnace, the quantity of carbon in the charge should not exceed a stoichiometric one and the same should be uniformly distributed throughout the volume of the furnace. However this condition is difficult to realize when utilizing briquetted charge with different bulk weights of individual components. This being the case, there are formed complex oxidic compounds on the furnace top, in particular silicates, when metal oxides interact with silica (primary slag formation).
One of the possible ways of intensifying reduction of metals is milling and subsequent briquetting of all the components of the charge. The mixing and briquetting of all the components of the charge does not preclude the possibility of slag formation, since together with improved kinetic conditions of metal reduction there grows a contacting surface of oxide forming thermodynamically stable chemical compounds.
To preclude slag formation in smelting silicon containing ferro alloys, it has been proposed to separately form briquettes from a mixture of sandy quartz with coal as disclosed in British Pat. No. 1051809.
However, these conditions contribute to extremely quick development of the reactions of forming gaseous silicon monoxide --(at 1727.degree. C. and P.sub.SiO =1.19 atm) and silicon carbide leading to the loss of silicon and to deposit formation in the smelting chamber of the furnace.
In addtion, due to the advancing reduction of silicon there arise conditions for participation thereof in reducing the alloying element in each reaction involving the formation of secondary silicates.
Also, known in the art is a process for producing silicon containing ferro alloys Fe-Si-Al and Fe-Si-Gr-Al disclosed in USSR Inventor's Certificate No. 273236. This process consists in a continuous one-stage carbothermic reducing the elements of the charge comprising quartzite and briquetted mixture of an alumina containing raw material with a carbonaceous reducing agent wherein a carbon content is 1.5-2 times as high as that sufficient for aluminium to be reduced.
In addition, as has been stated this process can be applied only for alloys Fe-Si-Al, wherein a content of aluminium is 25% and a content of silicon up to 20%. The realization of this method involves the following difficulties:
A continuous slagless process of smelting can not be realized on account of an indefinite carbon content contained in the whole of the charge with due regard for the reduction of silicon from quartzite.
Specifically, the excess of carbon in the charge eliminates the formation of slag but the furnace smelting chamber inevitably gets laid down with carbide deposits. And as a result smelting is to be stopped. A decrease in the charge carbon content beyond a certain amount eleminates the formation of deposits but causes the formation of slag and a decrease in reducing the elements into the alloy. As a consequence, the carbothermic one-stage process is now used for smelting but some ferro alloys, such as silicomaganese, silico-calcium, all cases being characterized by a higher or lower amount of slag formed in the process.
The smelting of silicocalcium is rather power-consuming --(the temperature of the process is more than 2000.degree. C.) and labour-consuming and has not undergone any major improvements during last 20-30 years. Because of the deposits which are formed in the furnace and which consist mainly of carbides and silicates, the smelting of silico-calcium is carried out during short intervals, for instance 3 or 4 months. Thus obtained alloys contain inclusions of a carbide slag and tend to disintegrate into powder on exposure to air.
Absence of adequate processes for producing silicon-containing ferro alloys restricts their application in metallurgy and foundry practice, thereby hindering the extensive usage of advanced production methods.
To overcome the above-described difficulties it is necessary to carry out the process so as to simultaneously meet the opposite requirements that there should be a certain excess of a carbon content to prevent the formation of silicates and a certain defficiency thereof to completely disintegrate the carbides.